1. Field of the Invention:
This invention relates to telephoto lenses of increased aperture ratio suited to photographic cameras or video cameras and, more particularly, to high-aperture telephoto lenses well corrected for high grade performance.
2. Description of the Related Art:
In outdoor or sports photography, because the subject is very often shot from a long distance with a fast shutter speed, very high demands have been made on the combination of long focal length and large aperture ratio in the taking lens.
In general, the bulk and size of the telephoto lens increases at a proportional rate to the increase in the focal length. For this reason, if the focusing method of moving the entire system as a whole is employed in a telephoto lens of increased focal length, not only is its total focusing movement increased, but also, the stress on the operating mechanism is rapidly increased. Hence there has been a drawback that quick and easy focusing becomes very difficult to perform. Also, because the lens system to be driven is very heavy, the perfection of centering is liable to break, thus giving rise to a serious problem that the image quality drops.
To avoid these problems, another focusing method by moving the lens system in part, or a rear one of the lens units, or the so-called rear-focus method, may be employed. In fact, a wide variety of forms of the rear-focus telephoto lens have been proposed.
There are superior advantages that use of the rear-focus method provides compared to the whole body-focus method. For example because the lens unit to be used has a relatively strong refractive power and is small and light, the total focusing movement is short, leaving room for shortening the minimum object distance. Moreover, focusing becomes easier and quicker. For example, Japanese Laid-Open Patent Application No. Sho 52-55639 discloses a technique of making focusing provision in one lens unit of the photographic objective.
In this document, however, the maximum aperture is as small as 5.6 in F-number. If the lens speed is increased to 2.0 or more in F-number with the use of the rear-focus method, on the other hand, the range of variation of aberrations with focusing is, in many cases, markedly increased.
Particularly spherical aberration in the terms of not lower than 5th order varies remarkably. As the object distance approaches the minimum, the spherical aberrations are largely under-corrected, worsening the image quality, and coma also starts to increase. To reduce the variation of these aberrations, one way is to design the preceding lens unit by using at least three positive lenses and two negative lenses. That is, passing through such a preceding lens unit, the light beam is made to smoothly coverage so that lesser aberrations of higher orders are produced. As a result, it becomes possible to afford some improvement of the stabilization of the aberration correction of the following or focusing lens unit throughout.
However, this technique cannot assist in the stabilization of the annular spherical aberration and coma, although the higher order spherical aberrations are maintained stable at an acceptable level. To solve this problem, it is advantageous to adopt one of the following measures.
(i) The floating technique is applied to the focusing lens unit; PA1 (ii) The design of the focusing lens unit is revised so that the aberrations it produces are suppressed in itself to a minimum; and PA1 (iii) The design of the lens unit that follows the focusing lens unit is adapted to eliminate the variation of the aberrations.
In (i), mention may be made of the previous proposal for a telephoto lens in, for example, Japanese Laid-open patent Application No. Sho 59-176717 where the rear-focus method is used in combination with means for moving two lens members in differential relation, or in a so-called floating means, to compensate for variation of aberrations with focusing. However, this technique, because of necessitating differentiation of the focusing movements of the two members from each other, has alternative drawbacks. For example, the structure of construction of the operating mechanism becomes very complicated that the tolerance for centering has to be set far tighter, and further the motor torque should be increased as compared with lens members moved at the same speed. In application of, for example, an automatic focus control device to such an operating mechanism, a great increase of the dimensions of the drive source or motor is called for.
In (ii), because the necessary number of lens elements is increased and the weight is increased, with respect to the automatic focus control device, it is likely that similar drawbacks to those of (i) will be encountered.
The use of (iii) permits the focusing lens unit to be constructed in a simpler form. Therefore, the mechanical structure also becomes simpler, and moreover the tolerance for centering may be set looser. Further, the automatic focus control device can even be applied with an advantage lessening of the motor torque, etc.
Nonetheless, in the art of high-aperture telephoto lenses, there have been a few previous proposals for designing the lens unit that follows the focusing lens unit with a chief aim to assist in the stabilization of the aberration correction over the focusing range. Of these, the telephoto lens in Japanese Patent Publication No. Sho 56-13926 (U.S. Pat. No. 4,176,913) comprises three lens units of positive, negative, positive power in this order from the front with the focusing provision made at the second one, while the aberrations of the first and second lens units are balanced out by a suitable design of the third lens unit.
With the type of telephoto lens in that publication, however, when the aperture ratio is increased, a serious problem arises that good stability of aberration correction can no longer be maintained throughout the focusing range.